A register in a microprocessor is generally selected as follows. The control unit of the microprocessor generates the address of the register to be selected and applies it to the address bus of the microprocessor. This address is transmitted to an address decoder which then selects the register relating to the address by activating the selection input of this register. The register can then be read or its contents can be modified. This register is, for example, an address register or an instruction register.
It sometimes happens that the contents of the registers of the microprocessor are accidentally modified when a mistake is made, or when there is a malfunction of the system in the microprocessor, especially during the initialization of the microprocessor.